Update JS dependencies (#12782)

- Update all dependencies
- Add explicit postcss dependency as dictated by postcss-loader
- Adapt for new postcss-loader syntax
- Move sourceMap options to top for consistency

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

semantic.json

@@ -1,5 +1,5 @@
 {
-  "version": "2.8.6",
+  "version": "2.8.7",
   "base": "node_modules/fomantic-ui",
   "paths": {
     "source": {